Imaging optical system and endoscope provided with imaging optical system

ABSTRACT

An imaging optical system includes a spherical or nearly spherical viewing port, an objective lens that includes an aspheric surface and is formed as a single lens element, and a solid-state image sensor that receives an image formed by the imaging optical system. Specified conditions are satisfied by the imaging optical system and the objective lens so that an in-focus image having low distortion, sufficient contrast, and formed by light rays of restricted angles of incidence is formed even for an object in contact with the viewing port. The specified conditions relate to features of the imaging optical system such as focal length and f-number of the imaging optical system, astigmatism and distortion of the imaging optical system, and pixel pitch of the solid-state image sensor. An endoscope that includes the imaging optical system is also disclosed.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims benefit of priority under 35 U.S.C. § 119of JP 2005-129,191, filed Apr. 27, 2005, the contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

Endoscopes have become widely used in the medical and industrial fields.A capsule endoscope that integrates the imaging optical system into acapsule that can be swallowed has been used in the medical field. Forinstance, Japanese Laid-Open Patent Application 2003-260024 disclosesone example of a capsule endoscope that includes an objective lens forforming an image of an object, a solid-state image sensor for convertingthe optical image into an electronic signal, and a light emitting diodefor illuminating the object, all of which are integrated within ahemispherical or nearly hemispherical transparent cover.

Additionally, Japanese Laid-Open Patent Application 2003-5031 disclosesan example in which a single aspheric lens element is used in theobjective optical system so that production costs can be reduced and theimaging optical system can be miniaturized, which in turn generallyenables producing a small electronic imaging device suitable for devicessuch as a cellular phone.

An imaging optical system that captures a good quality image that issmall and suitable for viewing with an endoscope, and that also can beproduced at a low cost, is demanded even in an endoscope wherein theimaging optical system is integrated into a hemispherical or nearlyhemispherical transparent cover at the tip of, for example, a capsuleendoscope. Unfortunately, however, if the imaging optical systemdescribed in Japanese Laid-Open Patent Application 2003-5031 were to beapplied to this type of endoscope, a good quality image that is suitablefor viewing with an endoscope would not be obtained although a reductionin cost could be achieved. The imaging optical system described inJapanese Laid-Open Patent Application 2003-5031 is designed as anoptical system for capturing images of objects generally in an outdoorsetting.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to an imaging optical system having aspherical or nearly spherical viewing port and further relates to anendoscope using such an imaging optical system. Additionally, thepresent invention relates to an imaging optical system that can obtain agood quality image when viewing through an endoscope, as well as to animaging optical system that can be made small and can be produced at lowcost even when the imaging optical system is used in an endoscope havinga spherical or nearly spherical viewing port.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given below and the accompanying drawings, whichare given by way of illustration only and thus are not limitative of thepresent invention, wherein:

FIG. 1 is a cross-sectional view of an imaging optical system forexplaining the nature of the present invention;

FIG. 2 shows aberrations related to explaining the present invention andparticularly shows astigmatism in terms of the curvatures of thesagittal and meridional image surfaces in relation to a Gaussian imagesurface;

FIG. 3 is a cross-sectional view showing the internal construction of acapsule endoscope;

FIG. 4 is a cross-sectional view of an imaging optical system accordingto Embodiment 1;

FIG. 5( a) shows astigmatism in terms of the curvatures of the sagittaland meridional image surfaces in relation to a Gaussian image surfaceaccording to Embodiment 1, and FIG. 5( b) shows distortion according toEmbodiment 1;

FIG. 6 shows a cross-sectional view of the diaphragm that serves as astop and the objective lens of Embodiment 1 aligned for connectingtogether;

FIG. 7 is a cross-sectional view of an imaging optical system accordingto Embodiment 2;

FIG. 8( a) shows astigmatism in terms of the curvatures of the sagittaland meridional image surfaces in relation to a Gaussian image surfaceaccording to Embodiment 2, and FIG. 8( b) shows distortion according toEmbodiment 2;

FIG. 9 is a cross-sectional view of an imaging optical system accordingto Embodiment 3;

FIG. 10( a) shows astigmatism in terms of the curvatures of the sagittaland meridional image surfaces in relation to a Gaussian image surfaceaccording to Embodiment 3, and FIG. 10( b) shows distortion according toEmbodiment 3;

FIG. 11( a) shows a cross-sectional view of the objective lens and itslens frame of Embodiment 3 connected together, and FIG. 11( b) shows aview from the image surface side of the objective lens of Embodiment 3;

FIG. 12 is a cross-sectional view of an imaging optical system accordingto Embodiment 4;

FIG. 13( a) shows astigmatism in terms of the curvatures of the sagittaland meridional image surfaces in relation to a Gaussian image surfaceaccording to Embodiment 4, and FIG. 13( b) shows distortion according toEmbodiment 4;

FIG. 14( a) shows a cross-sectional view of the objective lens and itslens frame of Embodiment 4 connected together, FIG. 14( b) shows anobject side view of the objective lens of Embodiment 4, and FIG. 14( c)shows an image side view of the objective lens of Embodiment 4;

FIG. 15 is a cross-sectional view of an imaging optical system accordingto Embodiment 5;

FIG. 16( a) shows astigmatism in terms of the curvatures of the sagittaland meridional image surfaces in relation to a Gaussian image surfaceaccording to Embodiment 5, and FIG. 16( b) shows distortion according toEmbodiment 5;

FIG. 17 is a cross-sectional view of an imaging optical system accordingto Embodiment 6;

FIG. 18( a) shows astigmatism in terms of the curvatures of the sagittaland meridional image surfaces in relation to a Gaussian image surfaceaccording to Embodiment 6, and FIG. 18( b) shows distortion according toEmbodiment 6;

FIG. 19 is a cross-sectional view of an imaging optical system accordingto Embodiment 7;

FIG. 20( a) shows astigmatism in terms of the curvatures of the sagittaland meridional image surfaces in relation to a Gaussian image surfaceaccording to Embodiment 7, and FIG. 20( b) shows distortion according toEmbodiment 7;

FIG. 21 is a cross-sectional view of the objective lens and the lensframe of Embodiment 7 connected together;

FIG. 22 is a cross-sectional view of an imaging optical system accordingto Embodiment 8;

FIG. 23( a) shows astigmatism in terms of the curvatures of the sagittaland meridional image surfaces in relation to a Gaussian image surfaceaccording to Embodiment 8, and FIG. 23( b) shows distortion according toEmbodiment 8;

FIG. 24 is a cross-sectional view of an imaging optical system accordingto Embodiment 9;

FIG. 25( a) shows astigmatism in terms of the curvatures of the sagittaland meridional image surfaces in relation to a Gaussian image surfaceaccording to Embodiment 9, and FIG. 25( b) shows distortion according toEmbodiment 9;

FIG. 26 is a cross-sectional view of an imaging optical system accordingto Embodiment 10;

FIG. 27( a) shows astigmatism in terms of the curvature of the sagittaland meridional image surfaces in relation to a Gaussian image surfaceaccording to Embodiment 10, and FIG. 27( b) shows distortion accordingto Embodiment 10;

FIG. 28 is a graph showing the aspheric displacement and the secondderivative of the aspheric displacement of the aspheric surface of theobjective lens of Embodiment 1;

FIG. 29 is a graph showing the aspheric displacement and the secondderivative of the aspheric displacement of the aspheric surface of theobjective lens of Embodiment 2;

FIG. 30 is a graph showing the aspheric displacement and the secondderivative of the aspheric displacement of the aspheric surface of theobjective lens of Embodiment 3;

FIG. 31 is a graph showing the aspheric displacement and the secondderivative of the aspheric displacement of the aspheric surface of theobjective lens of Embodiment 4;

FIG. 32 is a graph showing the aspheric displacement and the secondderivative of the aspheric displacement of the aspheric surface of theobjective lens of Embodiment 5;

FIG. 33 is a graph showing the aspheric displacement and the secondderivative of the aspheric displacement of the aspheric surface of theobjective lens of Embodiment 6;

FIG. 34 is a graph showing the aspheric displacement and the secondderivative of the aspheric displacement of the aspheric surface of theobjective lens of Embodiment 7;

FIG. 35 is a graph showing the aspheric displacement and the secondderivative of the aspheric displacement of the aspheric surface of theobjective lens of Embodiment 8;

FIG. 36 is a graph showing the aspheric displacement and the secondderivative of the aspheric displacement of the aspheric surface of theobjective lens of Embodiment 9; and

FIG. 37 is a graph showing the aspheric displacement and the secondderivative of the aspheric displacement of the aspheric surface of theobjective lens of Embodiment 10.

DETAILED DESCRIPTION

A viewing port having a spherical or nearly spherical shape is generallyused in endoscopes for medical use, such as capsule endoscopes, thatinclude a transparent viewing port in order to enable smooth insertioninto a body cavity. The phrase “spherical or nearly spherical” isintended to include not only slight variations from a true sphericalshape but also to include the viewing port being only part of aspherical shape generally, for example, forming a hemispherical ornearly hemispherical shape, which is descriptive of the invention asdescribed more specifically below. When such an endoscope is insertedinto a body cavity, the internal wall of the body cavity, which is theobject for observation, is pressed against the viewing port. Theperipheral part of the viewing port generally is in contact with theinternal wall of the body cavity while the center part, or tip, of theviewing port generally is not in contact with the internal wall of thebody cavity. In order to enable viewing of the internal wall of the bodycavity accurately and safely in this state, the internal wall of thebody cavity that is in contact with the peripheral part of the viewingport must be consistently brought into focus. An imaging optical systemthat includes an objective lens and a solid-state image sensor isarranged on the inside of the viewing port so that the central axis ofthe viewing port matches the optical axis of the imaging optical system.The objective lens optically forms an image of an object, which in thiscase is the internal wall of the body cavity. The solid-state imagesensor converts this image into an electronic signal and includes animage-receiving surface that is perpendicular to the optical axis of theimaging optical system and that intersects the optical axis at thecenter of an effective imaging area of the image-receiving surface. WhenIH is the distance of the farthest point from the optical axis withinthe effective imaging area, the internal wall of the body cavity that isin contact with the peripheral part of the viewing port is placed withina viewing range defined by the distances from the optical axis betweenIH/2 and IH on the effective imaging area.

Generally, when an imaging optical system is applied, as described inJapanese Laid-Open Patent Application 2003-5031, the object surfacedefined by the internal wall portion in contact with the viewing portsurface within the viewing range, as described above, is greatly curved.This is explained with reference to FIG. 1. The imaging optical systemof FIG. 1 includes an objective lens 2 and a solid-state image sensor 3,and the image-receiving surface of the solid-state image sensor 3 isarranged on the Gaussian image surface of the objective lens 2.Additionally, the optical axis X of the imaging optical system isarranged to match the central axis of a spherical or nearly sphericalviewing port 1. Beam A denotes a principal ray of the light beam thatforms an image at the farthest point or position at the distance IH fromthe center of the effective imaging area within the effective imagingarea of the image surface of the solid-state image sensor 3, and beam Bdenotes a principal ray of the light beam that forms an image at half ofthe distance IH of the farthest point or position from the center of theeffective imaging area, in other words, at the point or position at thedistance IH/2 from the center of the effective imaging area.Furthermore, an area where the point a at the intersection of beam A andthe viewing port and the point b at the intersection of beam B and theviewing port are connected along the viewing port is defined as theobject surface B within the viewing range determined by beam A and beamB. A plane that passes through the intersection point b and that isperpendicular to the optical axis X of the objective lens is defined asthe object surface A.

Generally, an imaging optical system as disclosed in Japanese Laid-OpenPatent Application 2003-5031 is constructed so that an object on anobject plane at infinity can form an image on the image-receivingsurface of the solid-state image sensor 3 that is arranged perpendicularto the optical axis of the objective lens 2. The image surface of aplanar object that is arranged close to the objective lens 2, such asthe object surface A, is also formed on a surface (image surface A,which is the image-receiving surface, in FIG. 1) that is perpendicularto the optical axis of the objective lens 2. Further, the image surfacethat corresponds to a curved object, such as the object surface B, whichis arranged close to the objective lens 2 is formed on the highly curvedsurface (image surface B in FIG. 1) that extends toward the object sidein relation to the image surface A that is perpendicular to the opticalaxis of the objective lens 2.

The position of the image-receiving surface of the solid-state imagesensor along the optical axis can be varied about the exact focusposition and still provide what is considered to be a focused image aslong as the variation from the exact focus position along the opticalaxis is less than or equal to a value of Δz that satisfies the followingCondition:

|Δz|=n·p/(tan(sin⁻¹(1/(2·Fno))))  Condition (1)

where

the product n·p is the limit of resolution of the solid-state imagesensor 3;

p is the pixel pitch of the solid-state image sensor 3;

n is a coefficient that is determined by the creation process of theluminance signal, with the value of this coefficient being determined bya circuit that processes the output of the solid state image sensor 3and an electronic signal that is input to the solid-state image sensor3; and

Fno is the effective f-number of the entire imaging optical system.

The coefficient n has a value of about 4 for a single-panel type,solid-state image sensor. Additionally, the depth of focus of theimaging optical system is equal to 2·|Δz|.

As described above, there is a need to obtain an image that is in focusby using an imaging optical system so that at least one of a sagittalimage surface and a meridional image surface that images the objectsurface B lies completely within a depth of focus in order for theimaging optical system to obtain the desired imaging of the objectsurface B for an endoscope having a spherical or nearly sphericalviewing port. In particular, there is a need for the optical systemformed of the transparent viewing port 1 and the objective lens 2 to usean objective lens 2 that results in at least one of the sagittal imagesurface and the meridional image surface related to an image of anobject on the viewing port surface being formed so that one of thefollowing Conditions related to 2·|Δz| is satisfied:

ΔS<2·|Δz|=8·p/tan(sin⁻¹(1/(2·Fno))))  Condition (2)

or

ΔM<2·|Δz|=8·p/(tan(sin⁻¹(1/(2·Fno))))  Condition (2)′

where

ΔS is the distance in the direction of the optical axis between pointsof a sagittal image surface of the image at distances IH/2 and IH fromthe optical axis of the imaging optical system along straight linesperpendicular to the optical axis, and the point at the distance IH isthe farthest point from the optical axis within an effective imagingarea of the image-receiving surface centered on the optical axis of theimaging optical system;

Δz, p, and Fno are defined above; and

ΔM is the distance in the direction of the optical axis between pointsof a meridional image surface of the image at distances IH/2 and IH fromthe optical axis of the imaging optical system along straight linesperpendicular to the optical axis, and the point at the distance IH isthe farthest point from the optical axis within an effective imagingarea of the image-receiving surface centered on the optical axis of theimaging optical system.

FIG. 2 shows the astigmatism of the imaging optical system in terms ofthe curvatures of the sagittal image surface (solid curved line) and themeridional image surface (dashed curved line) relative to a Gaussianimage surface (solid straight line). In FIG. 2, the aberration curvesextend to an image height of 1.07 mm, and the displacements (along theX-axis of the figure) are also expressed in millimeters. In FIG. 2, thepoints Smax, Smin, Mmax, and Mmin define image point locations of thesagittal and meridional image surfaces that are at different distancesin the direction of the optical axis relative to the Gaussian imagesurface, and these points are used to define ΔS and ΔM as follows:ΔS=Smax−Smin, ΔM=Mmax−Mmin.

When considering details of the construction of an imaging opticalsystem for an endoscope having a spherical or nearly sphericaltransparent viewing port, such as a capsule endoscope, the preferredconstruction is having the objective lens be a single lens element thatincludes an aspheric surface with a half-angle of view of fifty degreesor more and that is used with a solid-state image sensor having thecommon video signal format termed ‘CIF format’. The term “lens element”is herein defined as a single transparent mass of refractive materialhaving two opposed refracting surfaces, which surfaces are positioned atleast generally transversely of the optical axis of the imaging opticalsystem. Additionally, the pixel pitch of the solid-state image sensorshould maintain about a seven micrometer standard when standardizing thefocal length fL of an optical system that includes a transparent viewingport, and the effective aperture or f-number Fno of the imaging opticalsystem must be 3.7 or less in order to ensure adequate brightness withthe imaging optical system in combination with the use of a solid-stateimage sensor.

Accordingly, the focal depth (herein defined as 2·|Δz|, with |Δz| asgiven in Condition (1) above) of the imaging optical system in units ofmillimeters is as set forth in the following Condition (3) when thefeatures of the previous paragraph are applied:

2·|Δz|=0.39 mm   Condition (3).

Furthermore, with the objective lens being a single lens element, in animaging optical system as set forth above it is preferable that at leastone of the sagittal image surface or meridional image surface satisfiesa corresponding Condition (4) or (4)′, respectively, within the range ofthe focal depth 2·|Δz|:

ΔS/fL<0.4  Condition (4)

or

ΔM/fL<0.4  Condition (4)′

where

fL is the focal length of the optical system that includes a transparentviewing port, and

ΔS and ΔM are defined above.

FIG. 3 is a cross-sectional view showing the internal construction of acapsule endoscope. As shown in FIG. 3, the capsule endoscope 4 includesan objective lens 18 and an LED 25 or similar device on the inner sideof a transparent viewing port 16 that is sealed to a cover 17. Theobjective lens 18 is mounted on a lens frame 20 opposite the centerpart, or tip, of the viewing port 16, and an aperture stop diaphragm 19(that is integrally formed with the lens frame 20) is arranged at theobject-side surface of the objective lens 18. The lens frame 20 fitswithin a frame 21 mounted on a substrate 22 of the solid-state imagesensor 23, and the assembly is cemented in place. Also, a plurality ofLEDs 25 are mounted on a substrate 24, cemented in place, and fitted tothe frame 21. An image sensor, such as a CCD, CMOS, or similar sensor,is used as the solid-state image sensor 23. Connection sections 26, 29,and 31 electrically connect the substrate 22 to a drive processingcircuit 27, a memory circuit 28, and a wireless communication circuit30, all of which are powered by button-type batteries 32. An antenna 33is connected to the wireless communication circuit 30.

Ten embodiments of the present invention will now be discussed in detailwith further reference to the drawings and with reference to varioustables. In Tables 2-11 of Embodiments 1-10, respectively, the opticalsurfaces are labeled in numerical order under the heading “S” from theobject-side surface of the viewing port. Thus, the object-side surfaceof the viewing port is surface “1”, the image-side surface of theviewing port is surface “2”, and so forth until the last opticalsurface, which is an image surface, is denoted as surface “7” in Tables2-8 and as surface “8” in Tables 9-11. Various parameters of Embodiments1-10 are also shown in Table 1 below, which has been divided into Part 1for Embodiments 1-5 and Part 2 for Embodiments 6-10. In Table 1 below,each “x” in the bottom row indicates an embodiment where the meridionalimage surface has a large curvature such that ΔM exceeds 0.200 mm.

TABLE 1 Embodiment 1 Embodiment 2 Embodiment 3 Embodiment 4 Embodiment 5Radius of curvature of the flat surface INF INF INF INF INF side of theobjective lens Radius of curvature of the convex −0.68117 −0.68147−0.68115 −0.69682 −0.69418 surface side of the objective lens Thicknesson the optical axis of the 0.75829 0.88368 0.69033 0.56748 0.94003objective lens Eccentricity k of the aspheric surface 0 0 0 0 0 Asphericcoefficient A4 0.20276 −0.6981 0.12136 0.38565 −1.09502 Asphericcoefficient A6 0.70237 2.31451 1.03779 −0.39034 2.66146 Asphericcoefficient A8 0 −0.02542 0 0 0 Focal length of the entire imaging 1.0001.000 1.000 1.000 1.000 system Half-angle of view −55.527 −65.127−59.661 −52.472 −58.954 Angle of incidence to the image surface −26.702−23.223 −29.562 −32.178 −16.398 ΔS 0.095 0.127 0.048 0.133 0.141 ΔM0.059 0.258 0.191 0.196 0.308 x x Embodiment Embodiment 6 Embodiment 7Embodiment 8 Embodiment 9 10 Radius of curvature of the flat surface INF10.42047 INF INF INF side of the objective lens Radius of curvature ofthe convex −0.68917 −0.72818 −0.58869 −0.80769 −0.80835 surface side ofthe objective lens Thickness on the optical axis of the 0.63406 0.635671.50526 1.33135 1.32601 objective lens Eccentricity k of the asphericsurface 0 0 −0.838 −0.669 −0.632 Aspheric coefficient A4 0.10794 0.421070 0 0 Aspheric coefficient A6 0.8249 −0.4124 0 0 0 Aspheric coefficientA8 0 0 0 0 0 Focal length of the entire imaging 1.000 1.000 1.000 1.0001.000 system Half-angle of view −59.648 −52.472 −55.581 −54.994 −55.070Angle of incidence to the image surface −29.935 −31.447 −16.280 −14.375−15.723 ΔS 0.168 0.136 0.042 0.099 0.099 ΔM 0.081 0.220 0.430 0.0580.058 x x

In addition, the aspheric surface of the objective lens of each ofEmbodiments 1-10 is defined by the following Equation (A):

X=C·S²/[1+(1−(k+1)·C²·S²)^(1/2)]+A4·S⁴+A6S⁶+ . . . +An·S^(n)  Equation(A)

where

X is the length of a line segment from a point on the aspheric surfaceat a distance S from the optical axis measured perpendicular to theoptical axis to the plane that is tangent to the vertex of the asphericsurface;

C=1/R where R is the radius of curvature of the aspheric surface on theoptical axis;

k is the eccentricity of the aspheric surface; and

A4, A6 . . . An are aspheric coefficients.

Embodiments 1-10 of the present invention will now be individuallydescribed with further reference to the drawings and to various tables.

EMBODIMENT 1

Embodiment 1 is explained below with reference to FIGS. 4, 5(a), 5(b),and 6, as well as with reference to Table 2 below. FIG. 4 is across-sectional view of an imaging optical system according toEmbodiment 1. FIG. 5( a) shows astigmatism in terms of the curvatures ofthe sagittal image surface (shown by a solid curved line) and themeridional image surface (shown by a dashed curved line) in relation toa Gaussian image surface (shown by a solid straight line) according toEmbodiment 1, and FIG. 5( b) shows distortion (in %) according toEmbodiment 1. In FIGS. 5( a) and 5(b), the aberration curves extend toan image height of 1.03 mm, and in FIG. 5( a) the displacements (alongthe X-axis) are expressed in millimeters. Figures similar to FIGS. 5( a)and 5(b) will be used below to similarly describe Embodiments 2-10 ofthe present invention. FIG. 6 shows a cross-sectional view of thediaphragm that serves as an aperture stop and the objective lens ofEmbodiment 1 aligned for connecting them together. In addition,reference symbols common to FIGS. 3, 4, and 6 that are used to describethe construction of Embodiment 1 similarly describe the imaging opticalsystems of Embodiments 2-10 that will be described after Embodiment 1.Table 2 below shows optical design data of the imaging optical system ofEmbodiment 1. In Table 2, S is the surface number, RDY is the on-axisradius of curvature of this surface, THI is the on-axis surfaceseparation between this surface and the next higher numbered surface,and Nd and Vd are the refractive index and Abbe number, respectively,(both measured at the d-line) of the material that follows this surface.INF indicates an infinite radius of curvature related to a planarsurface. In this embodiment, the sixth surface (S6) is an asphericsurface with an eccentricity of zero, with only the asphericcoefficients A4 and A6 being non-zero, and with the values thereof beinggiven in Table 1 above.

TABLE 2 S RDY THI Nd Vd  8.4974 (object surface) 0.0000 1  8.4974 1.54501.51825 64.14 2  7.4159 7.4798 3 INF (diaphragm) 0.0000 4 INF 0.0400 5INF 0.7583 1.70235 70.00 6 −0.6812 (aspheric surface) 1.0882 7 INF(image surface) 0.0000

As shown in FIG. 4, the imaging optical system of Embodiment 1 includes,arranged in order from the object side, a transparent viewing port 16with a spherical surface, an objective lens 18 that is a singleplano-convex lens element, and a solid-state image sensor 23. The angleof view (i.e., the field angle) is 110°. The convex surface side of theobjective lens 18 is an aspheric surface, and a diaphragm 19 that servesas an aperture stop is arranged at the flat surface side. An imagesurface of an object surface B (hereinafter referred to simply as ‘theobject B’ in the descriptions of all the embodiments) that isdistributed along the surface of the viewing port 16 is formed in thevicinity of the image surface of the solid-state image sensor 23 withinthe effective imaging area of the image surface of the solid-state imagesensor 23. The center of the effective imaging area is centered on theoptical axis X of the imaging optical system. With the distance to thefarthest point in the effective imaging area from the center P of theeffective imaging area shown as IH in FIG. 4, the aspheric surface ofthe objective lens 18 has a curvature toward the periphery that issmaller than the on-axis curvature and has a form wherein the amount ofdeviation from the on-axis curvature increases between where a principalray CR1 that defines one edge of the periphery of the viewing field ofthe field of view range FV12 traverses the aspheric surface for imagingat the distance IH/2 from the point P and where a principal ray CR2 thatdefines the other edge of the periphery of the viewing field in thefield of view range FV12 traverses the aspheric surface for imaging atthe distance IH from the point P. This increase in the amount ofdeviation also pertains to Embodiments 2-10 that follow and theirdescriptions include very similar drawing figures.

The image surface of the object B that is distributed along the surfaceof the viewing port 16 in the field of view range FV12 is formed as anearly flat image surface with the sagittal image surface having a depthin the direction of the optical axis X of ΔS=0.095 mm and the meridionalimage surface having a depth in the optical axis direction of ΔM=0.059mm. Thus, favorable viewing ability is achieved in the image, includingin the periphery of the image.

In addition, the optically effective lens portion and the lens frame maybe formed as an integral unit because the objective lens 18 ofEmbodiment 1 can be manufactured as molded glass, which enables theassembly time of the imaging unit to be reduced and the precision ofassembly to be improved. For example, as shown in FIG. 6, a frame unit18 b may be formed with rotational symmetry about the optical axis O atthe periphery of the effective lens portion 18 a of the objective lens18. Protrusions 18 c are also formed in at least two places on thesurface of the object side of the frame unit 18 b. By so doing, theoptical axis of the objective lens 18 and the central axis of theaperture stop diaphragm 19 can be easily aligned by fitting theprotrusions 18 c into alignment holes 19 a arranged in the aperture stopdiaphragm 19.

EMBODIMENT 2

Embodiment 2 is explained below with reference to FIGS. 7, 8(a), and8(b), as well as with reference to Table 3 below. FIG. 7 is across-sectional view of an imaging optical system according toEmbodiment 2. FIGS. 8( a) and 8 (b) show the astigmatism and distortion,respectively, for Embodiment 2 in the same manner that FIGS. 5( a) and5(b) show the astigmatism and distortion, respectively, for Embodiment 1as previously described. In Embodiment 2, the sixth surface (S6) is anaspheric surface with an eccentricity of zero, with only the asphericcoefficients A4, A6, and A8 being non-zero, and with the values thereofbeing given in Table 1 above.

TABLE 3 S RDY THI Nd Vd  8.8624 (object surface) 0.0000 1  8.8624 1.61131.51825 64.14 2  7.7345 7.8011 3 INF (diaphragm) 0.0000 4 INF 0.0400 5INF 0.8837 1.70235 70.00 6 −0.6815 (aspheric surface) 1.0826 7 INF(image surface) 0.0000

Although the basic construction of the imaging optical system ofEmbodiment 2 is the same as Embodiment 1, the angle of view is set to130°. In the field of view range FV12 shown in FIG. 7, the asphericsurface of the objective lens 18 has a curvature toward the peripherythat is larger than the on-axis curvature, and has a form having a pointof inflection within the field of view range FV12. The image surface ofthe object B that is distributed along the surface of the transparentviewing port 16 in the field of view range FV12 is formed so that thesagittal image surface has a depth in the direction of the optical axisX of ΔS=0.127 mm and the meridional image surface has a depth in thedirection of the optical axis X of ΔM=0.258 mm. Favorable viewingability at the periphery of the image can be secured and focusadjustment also can be easily performed when the position of theimage-receiving surface of the solid-state image sensor 23 is arrangedwith the ability to be adjusted in the optical axis direction withfocusing being directed to focusing on the sagittal image surface thathas less depth in the optical axis direction than the meridional imagesurface.

There is a need to construct the imaging optical systems of endoscopesfor medical use with the ability to form images so that pathologicalchanges that occur on a biological surface will not be overlooked. Thisis accomplished by providing a wide-angle field of view of 100° or more(preferably 130° or more). On the other hand, in order to enable viewingof an image of the object B that is distributed along the surface of thenearly spherical viewing port 16 by a single lens element as in theimaging optical system of the present invention, a form of asphericsurface that contributes to image formation is determined byconcentrating on either one of the sagittal image surface or themeridional image surface, and the position of the image-receivingsurface of the solid-state image sensor 23 in the direction of theoptical axis X is preferably regulated in relation to the selected imagesurface. By so doing, a single lens element having an aspheric surfaceof a reasonable form that is easily produced can be achieved and focusadjustment is easily performed with a simple construction.

EMBODIMENT 3

Embodiment 3 is explained below with reference to FIGS. 9, 10(a), 10(b),11(a) and 11(b), as well as with reference to Table 4 below. FIG. 9 is across-sectional view of an imaging optical system according toEmbodiment 3. FIGS. 10( a) and 10(b) show the astigmatism anddistortion, respectively, for Embodiment 3 in the same manner that FIGS.5( a) and 5(b) show the astigmatism and distortion, respectively, forEmbodiment 1 as previously described. In FIGS. 10( a) and 10(b), theaberration curves extend to an image height of 1.10 mm, and in FIG. 10(a) the displacements (along the X-axis) are expressed in millimeters.Table 4 below shows optical design data of the imaging optical system ofEmbodiment 3 in the same manner as Table 2 above shows optical designdata of the imaging optical system of Embodiment 1. In Embodiment 3, thesixth surface (S6) is an aspheric surface with an eccentricity of zero,with only the aspheric coefficients A4 and A6 being non-zero, and withthe values thereof being given in Table 1 above.

TABLE 4 S RDY THI Nd Vd  9.0446 (object surface) 0.0000 1  9.0446 1.64451.51825 64.14 2  7.8934 7.9614 3 INF (diaphragm) 0.0000 4 INF 0.0400 5INF 0.6903 1.70235 70.00 6 −0.6812 (aspheric surface) 1.0811 7 INF(image surface) 0.0000

Although the basic construction of the imaging optical system ofEmbodiment 3 is the same as Embodiment 1, the angle of view is set to120°. Additionally, the objective lens 18 is the same as the objectivelens of Embodiment 1 with regard to it being a single plano-convex lenselement and having the same curvatures on the optical axis, that is, thesame on-axis curvatures. In the field of view range FV12 shown in FIG.9, the aspheric surface of the objective lens 18 has a curvature towardthe periphery that is smaller than the on-axis curvature and has a formsuch that the amount of deviation from the on-axis curvature increasestoward the periphery of the field of view. The image surface of theobject B that is distributed along the surface of the transparentviewing port 16 in the field of view range FV12 is formed so that thesagittal image surface has a depth in the direction of the optical axisX of ΔS=0.048 mm and the meridional image surface has a depth in thedirection of the optical axis X of ΔM=0.191 mm.

FIG. 11( a) shows a cross-sectional view of the objective lens and thelens frame of Embodiment 3 connected together, and FIG. 11( b) shows aview from the image surface side of the objective lens of Embodiment 3.As shown in FIGS. 11( a) and 11(b), a pair of frame units 18 b areformed symmetrically on opposite sides of the effective lens portion 18a of the objective lens 18 with a protrusion 18 c on a surface of eachof the pair of frame units 18 b. Furthermore, an aperture stop diaphragm19 and a lens frame 20 are integrally formed, the protrusions 18 c arefitted into alignment holes 20 a of the lens frame 20, and the diaphragm19 is cemented in place inside the lens frame 20 so as to tightlyconnect the effective lens portion 18 a and the diaphragm 19. Anoperation of assembling the objective lens 18 on the side of the lensframe 20 can be performed easily by using holding instruments such asforceps or a similar device. Furthermore, a central axis of the aperturestop diaphragm 19 can be easily aligned with the optical axis O of theobjective lens 18 by fitting the protrusions 18 c in the alignment holes20 a of the lens frame 20.

EMBODIMENT 4

Embodiment 4 is explained below with reference to FIGS. 12, 13(a),13(b), 14(a), 14(b), and 14(c), as well as with reference to Table 5below. FIG. 12 is a cross-sectional view of an imaging optical systemaccording to Embodiment 4. FIGS. 13( a) and 13(b) show the astigmatismand distortion, respectively, for Embodiment 4 in the same manner thatFIGS. 5( a) and 5(b) show the astigmatism and distortion, respectively,for Embodiment 1 as previously described. In FIGS. 13(a) and 13(b), theaberration curves extend to an image height of 1.04 mm, and in FIG. 13(a) the displacements (along the X-axis) are expressed in millimeters.Table 5 below shows optical design data of the imaging optical system ofEmbodiment 4 in the same manner as Table 2 above shows optical designdata of the imaging optical system of Embodiment 1. In Embodiment 4, thesixth surface (S6) is an aspheric surface with an eccentricity of zero,with only the aspheric coefficients A4 and A6 being non-zero, and withthe values thereof being given in Table 1 above.

TABLE 5 S RDY THI Nd Vd  6.5676 (object surface) 0.0000 1  6.5676 1.21621.52765 56.25 2  5.3514 5.3514 3 INF (diaphragm) 0.0000 4 INF 0.0400 5INF 0.5675 1.70235 70.00 6 −0.6968 (aspheric surface) 1.1734 7 INF(image surface) 0.0000

Embodiment 4 is an example of an imaging optical system suitable forbeing mounted in a small capsule endoscope. The imaging optical systemof Embodiment 4 includes, arranged in order from the object side, atransparent viewing port 16 having spherical surfaces, an objective lens18 that is a single plano-convex lens element, and a solid-state imagesensor 23. The imaging optical system of Embodiment 4 has an angle ofview of 104°. The convex surface side of the objective lens 18 is anaspheric surface, and an aperture stop diaphragm 19 is arranged at theplanar surface side. In accordance with capsule miniaturization, thecurvature of the viewing port 16 is larger relative to that of theimaging optical system of Embodiment 1, and for this reason, aberrationcorrection by variations from a spherical surface in the objective lens18 must be larger. In Embodiment 4, in the field of view range FV12shown in FIG. 12, the aspheric surface of the objective lens 18 has acurvature toward the periphery that is smaller than the on-axiscurvature and has a form such that the amount of deviation from theon-axis curvature increases toward the periphery of the field of view.The image surface of the object B that is distributed along the surfaceof the transparent viewing port 16 in the field of view range FV12 isformed so that the sagittal image surface has a depth in the directionof the optical axis X of ΔS=0.133 mm and the meridional image surfacehas a depth in the direction of the optical axis X of ΔM=0.196 mm. Inthis case, by the position of the image-receiving surface of thesolid-state image sensor 23 being adjustable in the optical axisdirection with focusing being directed to focusing on the sagittal imagesurface, favorable viewing ability at the periphery of the image can beachieved and focusing adjustment can be made easier.

FIG. 14( a) shows a cross-sectional view of the objective lens and itslens frame of Embodiment 4 connected together, FIG. 14( b) shows anobject side view of the objective lens of Embodiment 4, and FIG. 14( c)shows an image side view of the objective lens of Embodiment 4. A pairof frame units 18 b are formed symmetrically on opposite sides of theoptical axis O of the objective lens 18 at the periphery of theeffective lens unit 18 a of the objective lens 18. Furthermore,protrusions 18 c are formed on the object-side surfaces of these frameunits 18 b, and protrusions 18 d are formed on the image-side surfacesof these frame units 18 b. The protrusions 18 c fit into alignment holes19 a in an aperture stop diaphragm 19 so that the effective lens unit 18a and the aperture stop diaphragm 19 are held tightly together.Additionally, the aperture stop diaphragm 19 fits in the lens frame 20and is cemented in place to the inner side of the lens frame 20 with theprotrusions 18 d in contact with the lens frame 20.

EMBODIMENT 5

Embodiment 5 is explained below with reference to FIGS. 15, 16(a), and16(b), as well as with reference to Table 6 below. FIG. 15 is across-sectional view of an imaging optical system according toEmbodiment 5. FIGS. 16( a) and 16(b) show the astigmatism anddistortion, respectively, for Embodiment 5 in the same manner that FIGS.5( a) and 5(b) show the astigmatism and distortion, respectively, forEmbodiment 1 as previously described. Table 6 below shows optical designdata of the imaging optical system of Embodiment 5 in the same manner asTable 2 above shows optical design data of the imaging optical system ofEmbodiment 1. In Embodiment 5, the sixth surface (S6) is an asphericsurface with an eccentricity of zero, with only the asphericcoefficients A4 and A6 being non-zero, and with the values thereof beinggiven in Table 1 above.

TABLE 6 S RDY THI Nd Vd  5.9222 (object surface) 0.0000 1  5.9222 1.17501.52765 56.25 2  4.8255 4.8255 3 INF (diaphragm) 0.0705 4 INF 0.0000 5INF 0.9400 1.70235 70.00 6 −0.6942 (aspheric surface) 1.1773 7 INF(image surface) 0.0000

Although the basic construction of the imaging optical system ofEmbodiment 5 is the same as Embodiment 1, the angle of view is set to118°. The objective lens 18 is a single piano-convex lens element withthe convex surface being an aspheric surface. The image surface of theobject B that is distributed along the surface of the transparentviewing port 16 in the field of view range FV12 is formed so that thesagittal image surface has a depth in the direction of the optical axisX of ΔS=0.141 mm and the meridional image surface has a depth in thedirection of the optical axis X of ΔM=0.308 mm by having the asphericsurface of the objective lens 18 in the field of view range FV12 so asto have a larger curvature than the on-axis curvature and also by makingthe amount of deviation of curvature from the on-axis curvature so as toincrease the curvature toward the periphery of the field of view.Favorable viewing ability at the periphery of the image can be securedand focus adjustment also can be easily performed when the position ofthe image-receiving surface of the solid-state image sensor 23 isarranged with the ability to be adjusted in the optical axis directionwith focusing being directed to focusing on the sagittal image surfacethat has less depth in the optical axis direction than the meridionalimage surface.

EMBODIMENT 6

Embodiment 6 is explained below with reference to FIGS. 17, 18(a), and18(b), as well as with reference to Table 7 below. FIG. 17 is across-sectional view of an imaging optical system according toEmbodiment 6. FIGS. 18( a) and 18(b) show the astigmatism anddistortion, respectively, for Embodiment 6 in the same manner that FIGS.5( a) and 5(b) show the astigmatism and distortion, respectively, forEmbodiment 1 as previously described. Table 7 below shows optical designdata of the imaging optical system of Embodiment 6 in the same manner asTable 2 above shows optical design data of the imaging optical system ofEmbodiment 1. In Embodiment 6, the sixth surface (S6) is an asphericsurface with an eccentricity of zero, with only the asphericcoefficients A4 and A6 being non-zero, and with the values thereof beinggiven in Table 1 above.

TABLE 7 S RDY THI Nd Vd 17.1966 (object surface) 0.0000 1 17.1966 1.71971.51825 64.14 2 14.6171 4.2992 3 INF (diaphragm) 0.0000 4 INF 0.0500 5INF 0.6341 1.70235 70.00 6 −0.6892 (aspheric surface) 1.1806 7 INF(image surface) 0.0000

Embodiment 6 is an example of an imaging optical system that uses aspherical dome having a large radius of curvature as a transparentviewing port 16. The center of curvature of the dome is arranged on theoptical axis of a plano-convex objective lens 18, and the angle of viewis set to 120°. Furthermore, the convex surface of the objective lens 18is an aspheric surface. In Embodiment 6, the image surface of the objectB that is distributed along the surface of the viewing port 16 in thefield of view range FV12 is formed so that the depth ΔS in the directionof the optical axis X of the sagittal image surface is 0.168 mm and thedepth ΔM in the direction of the optical axis X of the meridional imagesurface is 0.081 mm by making the aspheric surface of the objective lens18 in the field of view range FV12 so as to have a smaller curvaturethan the on-axis curvature and also by making the amount of deviation ofcurvature from the on-axis curvature larger toward the periphery of theviewing field. Favorable viewing ability at the periphery of the imagecan be obtained and focus adjustment can also be easily performed whenthe position of the image-receiving surface of the solid-state imagesensor 23 is arranged with the ability to adjust the position in theoptical axis direction while directing focusing to the meridional imagesurface that has a depth ΔM that is less than the depth ΔS of thesagittal image surface in the optical axis direction.

EMBODIMENT 7

Embodiment 7 is explained below with reference to FIGS. 19, 20 (a),20(b), and 21, as well as with reference to Table 8 below. FIG. 19 is across-sectional view of an imaging optical system according toEmbodiment 7. FIGS. 20( a) and 20(b) show the astigmatism anddistortion, respectively, for Embodiment 7 in the same manner that FIGS.5( a) and 5(b) show the astigmatism and distortion, respectively, forEmbodiment 1 as previously described. Table 8 below shows optical designdata of the imaging optical system of Embodiment 7 in the same manner asTable 2 above shows optical design data of the imaging optical system ofEmbodiment 1. In Embodiment 7, the sixth surface (S6) is an asphericsurface with an eccentricity of zero, with only the asphericcoefficients A4 and A6 being non-zero, and with the values thereof beinggiven in Table 1 above.

TABLE 8 S RDY THI Nd Vd  5.5649 (object surface) 0.0000 1  5.5649 1.21571.52765 56.25 2  5.3492 5.3492 3 INF (diaphragm) 0.0000 4 INF 0.0000 510.4205 0.6357 1.70235 70.00 6 −0.7282 (aspheric surface) 1.1481 7 INF(image surface) 0.0000

In Embodiment 7, an objective lens 18 is constructed as a single lenselement having positive refractive power, a non-zero curvature on bothsurfaces, and with the image-side surface being an aspheric surface. Theangle of view is set to 104°. Furthermore, the image surface of theobject B that is distributed along the surface of the transparentviewing port 16 in the field of view range FV12 is formed so that thedepth ΔS in the direction of the optical axis X of the sagittal imagesurface is 0.136 mm and the depth ΔM in the direction of the opticalaxis X of the meridional image surface is 0.22 mm by making the asphericsurface in the field of view range FV12 so as to have a smallercurvature than the on-axis curvature and also by making the amount ofdeviation of the curvature from the on-axis curvature become largertoward the periphery of the viewing field. Favorable viewing ability atthe periphery of the image can be obtained and focus adjustment can alsobe easily performed when the position of the image-receiving surface ofthe solid-state image sensor 23 is arranged with the ability to adjustit in the optical axis direction and directing focusing to the sagittalimage surface that has a depth ΔS that is less than the depth ΔM of themeridional image surface in the optical axis direction. Moreover, theobject-side surface of the objective lens 18 is constructed so as tohave positive refractive power, so that the effective diameter of theobjective lens 18 can be smaller as a result of lowering the beam heightin the field of view range FV12 on the aspheric surface. Consequently,technical properties of the objective lens can be improved by reducingthe amount of deviation from the on-axis curvature in the periphery ofthe aspheric surface.

FIG. 21 is a cross-sectional view of the objective lens and the lensframe of Embodiment 7 connected together. In the objective lens 18, apair of frame units 18 b are formed symmetrically on opposite sides ofthe optical axis O of the objective lens 18 at the periphery of theeffective lens unit 18 a of the objective lens 18. Additionally, aprotrusion 18 c is formed rotationally symmetrically about the opticalaxis O on the object-side surface of the effective lens unit 18 a.Furthermore, thin arm units 18 d that can be bent inwardly and outwardlyand that end in flanges 18 e extend to the image side from the pair offrame units 18 b. An aperture stop diaphragm 19 and a lens frame 20 areformed integrally as one unit, and the protrusion 18 c fits into analignment recess 20 a. A flange 20 b is arranged on the rim of the imageside end of the lens frame 20, and the objective lens 18 is fixed on theinner side of the lens frame without adhesive by latching the flanges 18e and 20 b in abutting contact. Assembling the objective lens 18 ontothe inner side of the lens frame 20 is done by inserting the objectivelens 18 into the inner side of the lens frame 20 until the protrusion 18c fits into the alignment recess 20 a while the assembly is held by atool such as a forceps, which avoids troublesome assembly procedures.

EMBODIMENT 8

Embodiment 8 is explained below with reference to FIGS. 22, 23(a), and23(b), as well as with reference to Table 9 below. FIG. 22 is across-sectional view of an imaging optical system according toEmbodiment 8. FIGS. 23( a) and 23(b) show the astigmatism anddistortion, respectively, for Embodiment 8 in the same manner that FIGS.5( a) and 5(b) show the astigmatism and distortion, respectively, forEmbodiment 1 as previously described. Table 9 below shows optical designdata of the imaging optical system of Embodiment 8 in the same manner asTable 2 above shows optical design data of the imaging optical system ofEmbodiment 1. In Embodiment 8, the sixth surface (S6) is an asphericsurface with a non-zero eccentricity, with all other asphericcoefficients (i.e., A2, A4, A6, etc.) being zero, and with the value ofthe eccentricity k being given in Table 1 above.

TABLE 9 S RDY THI Nd Vd  8.1970 (object surface) 0.0000 1  8.1970 1.63941.58874 30.49 2  6.5576 6.5576 3 INF (diaphragm) 0.0447 4 INF 0.0000 5INF 1.5053 1.58874 30.49 6 −0.5887 (aspheric surface) 0.0000 7 INF1.1333 8 INF (image surface) 0.0000

The imaging optical system of Embodiment 8 includes, arranged in orderfrom the object side, a transparent viewing port 16 with a sphericalsurface, an objective lens 18 that is a single plano-convex lenselement, and a solid-state image sensor 23. The angle of view (i.e., thefield angle) is 111°. The convex surface of the objective lens 18 is anellipsoidal surface having rotational symmetry about the optical axis.An aperture stop diaphragm is arranged on the planar side of theobjective lens 18.

The shape of the ellipsoidal surface of the objective lens 18 isdetermined by Equation (A) above using only the aspheric coefficient k,which is the eccentricity. That is, higher order aspheric coefficients,such as A4, A6, A8, etc., are zero in Equation (A) for this Embodiment8. In this case, the eccentricity preferably satisfies the followingCondition (5), and in Embodiment 8, the eccentricity k specificallyequals −0.8380:

−0.6<k<−0.85  Condition (5).

The convex surface of the objective lens 18 of Embodiment 8 is anellipsoid surface that is easy to trace, not an aspheric surface with acomplicated form, so that technical properties of the objective lens canbe improved. The ellipsoid surface has a curvature that decreases in thedirection away from the optical axis, and the amount of deviation fromthe on-axis curvature in the periphery of the aspheric surface increasestoward the periphery of the viewing field in the field of view rangeFV12.

It is necessary to consider the gate when designing a mold for castingplastic resin in order to properly form the objective lens of plasticmaterial. In order to make the design related to the gate easier, thethickness D of the objective lens on the optical axis preferablysatisfies the following Condition:

1.51>D/fL>0.94  Condition (6)

where

D is the thickness of the objective lens at the center of the objectivelens; and

fL is the focal length of the entire imaging optical system.

In Embodiment 8, the thickness D of the objective lens at the center ofthe objective lens is 1.5 mm.

Along with the convex surface having an ellipsoidal shape, the thicknessD of the objective lens 18 at its center, which is on the optical axis,is made relatively large, so that the angle of incidence Tw of aprincipal ray of the objective lens 18 can satisfy the followingCondition (7) in relation to the image-receiving surface of the imagesensor 23 for the light beam that forms an image on the image-receivingsurface in the field of view range FV12:

|Tw|<16.5°  Condition (7).

Having the angle of incidence Tw not satisfy Condition (7) isundesirable because in that case part of the incident light beam will beinterrupted by the rim surrounding the image sensor, which decreases theamount of light passing to the image sensor. Also, too large an angle ofincidence may result in light that should relate to a single pixelspanning two or more pixels on the image-receiving surface. Accordingly,the absolute value of the angle of incidence Tw of the principal rayrelated to the maximum image height position on the image-receivingsurface of the image sensor 23 is 16.3° in Embodiment 8.

In addition, when the light beam is interrupted by a rim or similarstructure, and the principal ray of an image at the maximum image heightposition on the image-receiving surface of the image sensor 23 cannot bedefined, the angle of incidence may be defined by considering a lightray at a central position of the light beam to be a principal ray.

The image surface of the object B that is distributed along the surfaceof the transparent viewing port 16 in the field of view range FV12 isformed so that the depth ΔS in the direction of the optical axis X ofthe sagittal image surface is 0.042 mm and the depth ΔM in the directionof the optical axis X of the meridional image surface is 0.43 mm. Thedepth ΔM in the direction of the optical axis X of the meridional imagesurface does not satisfy Condition (1) above in this case; however, whenthe position of the image-receiving surface of the solid-state imagesensor 23 is arranged so that focusing is directed to focusing of thesagittal image surface that has a depth ΔS equal to 0.042, favorableviewing ability at the periphery of an image can be ensured and thefocus adjustment can also be performed easily.

EMBODIMENT 9

Embodiment 9 is explained below with reference to FIGS. 24, 25(a), and25(b), as well as with reference to Table 10 below. FIG. 24 is across-sectional view of an imaging optical system according toEmbodiment 9. FIGS. 25( a) and 25(b) show the astigmatism anddistortion, respectively, for Embodiment 9 in the same manner that FIGS.5( a) and 5(b) show the astigmatism and distortion, respectively, forEmbodiment 1 as previously described. Table 10 below shows opticaldesign data of the imaging optical system of Embodiment 9 in the samemanner as Table 2 above shows optical design data of the imaging opticalsystem of Embodiment 1. In Embodiment 9, the sixth surface (S6) is anaspheric surface with a non-zero eccentricity, with all other asphericcoefficients (i.e., A2, A4, A6, etc.) being zero, and with the value ofthe eccentricity k being given in Table 1 above.

TABLE 10 S RDY THI Nd Vd  7.2115 (object surface) 0.0000 1  7.21151.9970 1.58874 30.49 2  5.2145 5.2145 3 INF (diaphragm) 0.0666 4 INF0.0000 5 INF 1.3314 1.81078 40.88 6 −0.8077 (aspheric surface) 0.0000 7INF 1.1583 8 INF (image surface) 0.0000

Although the basic construction of the imaging optical system ofEmbodiment 9 is the same as Embodiment 8, the angle of view is set to110°. In Embodiment 9, the convex surface of the plano-convex objectivelens 18 is an ellipsoidal surface having rotational symmetry about theoptical axis, and the eccentricity of the convex surface is −0.6690. Theellipsoidal surface of the objective lens 18 in the field of view rangeFV12 has a larger curvature than the on-axis curvature, and theellipsoidal surface has a curvature with the amount of deviation fromthe on-axis curvature becoming larger toward the periphery of theperiphery of the viewing field. The image surface of the object B thatis distributed along the surface of the transparent viewing port 16 inthe field of view range FV12 is formed so that the depth ΔS in thedirection of the optical axis X of the sagittal image surface is 0.099mm and the depth ΔM in the direction of the optical axis X of themeridional image surface is 0.058 mm by making the aspheric surface inthe field of view range FV12. Favorable viewing ability at the peripheryof the image can be obtained and focus adjustment can also be easilyperformed when the position of the image-receiving surface of thesolid-state image sensor 23 is arranged with the ability to adjust it inthe optical axis direction and directing focusing to the meridionalimage surface in this case. In addition, the thickness of the objectivelens at its center on the optical axis is 1.33 mm and the absolute valueof the angle of incidence Tw related to the maximum image heightposition on the image-receiving surface of the image sensor 23 is 14.4°.

EMBODIMENT 10

Embodiment 10 is explained below with reference to FIGS. 26, 27(a), and27(b), as well as with reference to Table 11 below. FIG. 26 is across-sectional view of an imaging optical system according toEmbodiment 10. FIGS. 27( a) and 27(b) show the astigmatism anddistortion, respectively, for Embodiment 10 in the same manner thatFIGS. 5( a) and 5(b) show the astigmatism and distortion, respectively,for Embodiment I as previously described. Table 11 below shows opticaldesign data of the imaging optical system of Embodiment 10 in the samemanner as Table 2 above shows optical design data of the imaging opticalsystem of Embodiment 1. In Embodiment 10, the sixth surface (S6) is anaspheric surface with a non-zero eccentricity, with all other asphericcoefficients (i.e., A2, A4, A6, etc.) being zero, and with the value ofthe eccentricity k being given in Table 1 above.

TABLE 11 S RDY THI Nd Vd  7.0125 (object surface) 0.0000 1  7.01251.4025 1.58874 30.49 2  5.6100 5.6100 3 INF (diaphragm) 0.0383 4 INF0.0000 5 INF 1.3260 1.81078 40.88 6 −0.8084 (aspheric surface) 0.0000 7INF 1.1576 8 INF (image surface) 0.0000

The basic construction of the imaging optical system of Embodiment 10 isthe same as Embodiment 8, and the angle of view is set to 110°, the sameas in Embodiment 9. The convex surface of the plano-convex objectivelens 18 is an ellipsoidal surface having rotational symmetry about theoptical axis, and the eccentricity of the convex surface is −0.6320. Theimage surface of the object B that is distributed along the surface ofthe transparent viewing port 16 in the field of view range FV12 isformed so that the depth ΔS in the direction of the optical axis X ofthe sagittal image surface is 0.099 mm and the depth ΔM in the directionof the optical axis X of the meridional image surface is 0.058 mmbecause the ellipsoidal surface of the objective lens 18 has a largercurvature off-axis than its on-axis curvature, and the amount ofdeviation from its on-axis curvature increases toward the periphery ofthe viewing field in the field of view range FV12.

In this case, favorable viewing ability at the periphery of the imagecan be obtained and focus adjustment can also be easily performed whenthe position of the image-receiving surface of the solid-state imagesensor 23 is arranged with the ability to adjust it in the optical axisdirection and directing focusing to the meridional image surface, inthis case based on ΔM being less than ΔS. Furthermore, the technicalproperties of the objective lens are improved by having a smaller amountof deviation from the on-axis curvature in the field of view range FV12by setting the aspheric coefficient k to be near the lower limit of theCondition (5) above instead of having a larger on-axis curvature of theellipsoidal surface in comparison with Embodiment 8. In addition, thethickness of the objective lens at its center on the optical axis is1.32 mm and the absolute value of the angle of incidence Tw related tothe maximum image height position on the image-receiving surface of theimage sensor 23 is 15.7° in Embodiment 10.

Generally, it would be expected that each imaging optical system ofEmbodiments 1 through 10 would be constructed with an optical axiscoincident with the optical axis of the objective lens 18 by all theoptical structures, including the transparent viewing port, beingrotationally symmetric about the optical axis of the objective lens 18.However, even if the center of the viewing port 16 and the optical axisof the objective lens 18 do not coincide, efficiencies of the presentinvention can be achieved with each of Embodiments 1 through 10 bysatisfying Conditions (2) and (4) above. Similarly, an endoscope may beconsidered to have an optical axis that is the optical axis of theimaging optical system used in the endoscope whether or not the opticalaxis of the objective lens is precisely coincident with the optical axisof the imaging optical system.

As explained above, the desired imaging characteristics of the presentinvention relate particularly to the shape of the aspheric surface ofthe objective lens 18 in each of Embodiments 1 through 10. Therefore, adetailed explanation in regards to the shape of these aspheric surfacesfollows. This explanation will make reference to FIGS. 28 through 37that show graphs of the aspheric displacement, that is, the amount ofdeviation from a spherical shape of these aspheric surfaces. In each ofFIGS. 28 through 37, the aspheric displacement Sag(h) is measured alonga line parallel to the optical axis of the objective lens 18 and betweena point on a reference spherical surface having the radius of curvaturethat is equal to the radius of curvature R that the aspheric surface hason the optical axis of the objective lens 18 and a point on the asphericsurface. In particular, the aspheric displacement Sag(h) is given by thefollowing Equation (B):

[Aspheric displacement Sag(h)]=−1·[Saga(h)−Sags(h)]  Equation (B)

where

Saga(h) is the coordinate value in the optical axis direction of thepoint on the aspheric surface at a beam height h; and

Sags(h) is the coordinate value in the optical axis direction of thepoint on the spherical surface at the beam height h.

The beam height h is within the range 0<h<IH. In FIGS. 28-37, the leftvertical axis is the scale for aspheric displacement Sag(h) and theright vertical axis is the scale for the second derivative of theaspheric displacement Sag(h), which is denoted as Sag″(h).

As shown in FIGS. 28-37, the aspheric displacement Sag(h) is positivethroughout except in Embodiments 2 and 5 (FIGS. 29 and 32,respectively). In other words, except in Embodiments 2 and 5, theaspheric surfaces have larger curvatures off-axis than the correspondingreference spherical surfaces and serve to correct negative imagedistortion. When a lens surface has a curved shape similar to thereference spherical surface, it is difficult to minimize distortion inthe range of beam height h from IH/2 to IH and it is also difficult tosatisfy Condition (4) above. On the other hand, when distortion for aflat surface object is corrected completely, the image surface becomeshighly curved, as shown by image surface B in FIG. 1, and the objectivesof the present invention cannot be achieved.

Considering the second derivative of the aspheric displacement Sag″(h),FIGS. 28 through 37 show upwardly concave curves for the secondderivative, related to correcting negative distortion, as compared to aspherical surface where the second derivative would be a straight lineindicating a constant positive second derivative. In the case ofEmbodiments 2 and 5, although a negative value is included part of theway, the change from a negative to a positive value is shown within therange of the beam height h between IH/2 and IH, with the distance IHbeing previously defined. This change is along upwardly concave curves,and the operation for correcting negative distortion from a negative toa positive direction is generated within the range of the beam heightbetween IH/2 and IH in this manner. In general, the efficiencies of thepresent invention can be achieved if there is at least a range of beamheights h where IH/2<h<IH where the following Condition is satisfied:

−d²Saga(h)/dh²+d²Sags(h)/dh²  Condition (8)

where

Saga(h), Sags(h), and h are defined as set forth previously.

Furthermore, there is a technique for determining the desired shape ofthe aspheric surface based on paraxial focal lengths defined inperpendicular directions for curvatures at specific beam heights.Distortion in the sagittal direction and in the meridional direction canbe corrected by satisfying the following Condition:

0.63<[fx·(IH)]/[fy·(IH)]<1  Condition (9)

where

fx is the paraxial focal length in the direction of closer focus relatedto the curvature at the beam height IH, with the distance IH being aspreviously defined; and

fy is the paraxial focal length in the direction of farther focusrelated to the curvature at the beam height IH, with the distance IHbeing as previously defined.

When the quantity fx(IH)/fy(IH) is less than or equal to the lower limitof Condition (9) above, the shape of the aspheric surface approachesthat of a spherical surface, making it difficult to minimize distortionin the range of beam heights h from b equals IH/2 to IH. Also, itbecomes difficult to satisfy Condition (4) above. Moreover, when thefx(IH)/fy(IH) is equal to one, distortion in relation to a flat objectsurface is corrected completely, but the distortion for an image surfaceB as shown in FIG. 1 above becomes excessive.

Furthermore, there is distortion in optical systems generally, anddistortion is particularly prominent in optical systems having a wideangle of view, such as endoscopes. However, enlarging the lens diameterfor correction is disadvantageous because there is commonly a need tomake the optical system as small as possible, and therefore generallydistortion is not well corrected. In contrast, however, with anendoscope having a transparent viewing port as in the present invention,it is possible to more easily deal with distortion based on the shapesof the objective lens surfaces and the outer surface of the transparentviewing port. Accordingly, when the object surface shape is defined bybeing immediately adjacent the surface of the viewing port, a favorableimage as to distortion (particularly, in the field of view range FV12)can be obtained when the imaging optical system satisfies the followingCondition:

−20%<ΔD<26%  Condition (10)

where

ΔD is the amount of distortion of an image formed by the imaging opticalsystem as determined at the distances IH and IH/2 from the optical axisalong straight lines perpendicular to said optical axis, with thedistances IH and IH/2 being as previously defined.

When the lower limit of Condition (10) is not satisfied, the image iscompressed unnaturally due to barrel distortion becoming large.Furthermore, when the upper limit of Condition (10) is not satisfied,pincushion distortion becomes too large, so that the image expands andappears to be unnaturally stretched.

A method for checking by a simple technique whether or not an imagingoptical system satisfies Conditions (2) and (4) above follows. Forexample, a white and black line pair is placed at the object pointposition of the transparent viewing port surface that has an imageforming relationship with the image at the image position at a distance4·IH/5 from the center of the effective imaging area of the solid-stateimage sensor so that the white and black line pair lie in a horizontaldirection on the image-receiving surface of the solid-state imagesensor, and a corresponding image is displayed on the display surface ofa display device. The width of the white and black line pair Ra, inmillimeters (mm), that provides ten per cent contrast of the white lineand black line is given by the following Equation (C):

Ra=0.028/(βlocal)  Equation (C)

where

Ra, as recited above, is the width (in mm) of the white and black linepair; and

β local is the magnification of the image position at a distance 4·IH/5from the center of the effective imaging area of a solid-state imagesensor of CIF format.

In each of Embodiments 1-10 described above, an appropriate value of βlocal is approximately 0.24. Using the value 0.24 in Equation (C) above,a value of Ra equal to 0.12 mm is determined. This value represents athreshold value for Conditions (2) and (4) above being satisfied. Inother words, when an imaging optical system satisfies the followingCondition (11), it will, generally speaking, also satisfy Conditions (2)and (4) above:

Ra>0.12 mm  Condition (11).

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention. Rather, the scopeof the invention shall be defined as set forth in the following claimsand their legal equivalents. All such modifications as would be obviousto one skilled in the art are intended to be included within the scopeof the following claims.

1-7. (canceled)
 8. An endoscope including an optical axis, the endoscopecomprising: a spherical or nearly spherical transparent viewing port; anobjective lens that includes at least one aspheric surface and thatoptically forms an image of an object, said objective lens being formedas a single lens element; and a solid-state image sensor for convertingsaid image into an electronic signal; wherein the following condition issatisfied−0.6<k<−0.85 where k is the eccentricity of said aspheric surface; andthe shape of said aspheric surface is defined according to the followingequationX=C·S²/[1+(1−(k+1)·C²·S²)^(1/2)] where X is the length of a line segmentfrom a point on said aspheric surface at a distance S from an axis ofsymmetry of said objective lens as measured perpendicular to said axisof symmetry to the plane that is tangent to the vertex of said asphericsurface; and C=I/R, where R is the radius of curvature of said asphericsurface on said optical axis.
 9. The endoscope according to claim 8,wherein: said solid-state image sensor includes an image-receivingsurface for receiving said image, said image-receiving surface beingperpendicular to said optical axis and intersecting said optical axis atthe center of an effective imaging area of said image-receiving surface;and the following conditions are satisfied:1.51>D/fL>0.940°<|Tw|<16.5° where D is the thickness of said objective lens along itsaxis of symmetry; fL is the focal length of the entire imaging opticalsystem; and Tw is the angle of incidence of a principal ray of saidobjective lens on said image-receiving surface at the farthest pointfrom said optical axis when said image is formed on said image-receivingsurface.
 10. (canceled)
 11. The endoscope according to claim 8, whereinsaid viewing port, said objective lens, and said image sensor form animaging optical system, said image is formed along an optical axis ofsaid objective lens, and said image sensor defines an effective imagingarea centered on said optical axis, and the following condition issatisfied−20%<ΔD<26% where ΔD is the amount of distortion of said image asdetermined at portions of said image about points of an image surface ofsaid image at distances IH/2 and IH from said optical axis alongstraight lines perpendicular to said optical axis, and the point at thedistance IH is the farthest point from said optical axis within saideffective imaging area.
 12. The endoscope according to claim 8, whereinsaid viewing port, said objective lens, and said image sensor form animaging optical system, said image is formed along an optical axis ofsaid objective lens, and said image sensor defines an effective imagingarea centered on said optical axis, and the following condition is alsosatisfied:w/2>50° where w is the field angle of said imaging optical system forsaid effective imaging area.
 13. The endoscope according to claim 8, andfurther including: an imaging optical system including an optical axisthat includes a spherical or nearly spherical transparent viewing port,an objective lens that includes at least one aspheric surface, is formedas a single lens element, has positive refractive power, and opticallyforms along said optical axis an image of an object viewed through saidtransparent viewing port; and a display device for displaying an imageproduced from the image received by said solid-state image sensor;wherein said solid-state image sensor includes an image-receivingsurface for receiving said image, said image-receiving surface beingperpendicular to said optical axis, and said optical axis intersectingsaid image-receiving surface at the center of an effective imaging area;and the following condition is satisfied:Ra>0.12 mm where Ra is the width, in mm, of a white and black line pairhaving ten per cent contrast when said white and black line pair formssaid object and said white and black line pair are imaged on saidimage-receiving surface at a distance 4·IH/5 from said optical axis andIH is the distance of the farthest point from said optical axis withinsaid effective imaging area.